MITOCHONDRIAL RESPIRATORY CONTROL: BIOCHEMICAL, PHYSIOLOGICAL, AND PATHOLOGICAL ASPECTS

Effects of proton motive force on the structure and dynamics of bovine cytochrome C oxidase in phospholipid vesicles

A conventional method for reconstituting cytochrome c oxidase (CcO) into phospholipid vesicles (COV) has been modified to permit resonance Raman (RR) analysis in the presence and absence of proton motive force (ΔμH(+)). The COV has an average diameter of 20 nm and contains one CcO molecule within a unified orientation with CuA located outside the COV. The process of generation of ΔμH(+) across the membrane was monitored spectrophotometrically with rhodamine123 dye. The COV exhibits a respiratory control ratio (RCR) value of >30 and is tolerant to RR measurements with 10 mW laser illumination for 60 min at 441.6 nm. Structural perturbations at the heme sites caused by incorporation into vesicles were clarified by spectral comparisons between solubilized CcO and COV. Absorption spectroscopy revealed that the rate of electron transfer from cytochrome c to O2 is reduced significantly more in the presence of ΔμH(+) than in its absence. RR spectroscopic measurements indicate that CcO in COV in the "respiratory-controlled" state adopts a mixed-valence state (heme a(2+) and heme a3(3+)). This study establishes a supramolecular model system for experimentally examining the energy conversion protein machinery in the presence of ΔμH(+).

Effects of cold acclimation and palmitate on energy coupling in duckling skeletal muscle mitochondria

Gastrocnemius subsarcolemmal and intermyofibrillar mitochondria were isolated from 5-week-old cold-acclimated and thermoneutral control ducklings. In vitro respiration (polarography) and ATP synthesis (bioluminescence) were determined at 25 degrees C. Subsarcolemmal mitochondria showed a higher state 4 respiration and lower respiratory control and ADP/O ratio in cold-acclimated than in thermoneutral ducklings. Palmitate decreased the rate of ATP synthesis in both mitochondrial populations to about 30% of maximal but failed to abolish this process even at high concentrations. It is suggested that both expensive ATP synthesis and increased ATP hydrolysis could contribute synergistically to muscle non-shivering thermogenesis in cold-acclimated ducklings.

Renal tubular sodium reabsorption and oxygen consumption in the hypothyroid dog

The degree to which an insufficiency of thyroid hormone (TH) affects renal tubular sodium reabsorption (RNa) and oxygen consumption (QO2) in vivo, was investigated in a mature mammalian kidney. The results suggest that TH influences renal QO2 as a result of alterations in renal hemodynamics and the renal tubular handling of sodium. Changes in GFR played a major role in the regulation of RNa and QO2. This is suggested by the comparison of hypothyroid animals with and without renal compensatory hypertrophy. In the latter group a twofold lower GFR was associated with parallel changes in RNa and QO2. However, in going from euthyroid (E) to hypothyroid (H) conditions, the decrease in RNa of 31 +/- 4% was greater than the reduction in GFR of 25 +/- 3% (p less than .05). Thus, a decrease in RNa of 6 +- 2% under hypothyroid conditions could not be attributed to the changes in GFR. The normal renal stoichiometry between RNa and QO2 in hypothyroid animals indicates that the above changes in RNa were accompanied by comparable changes in QO2. The decrease in RNa in hypothyroid animals was not associated with a significant change in kidney weight (E:48.2 +/- 4.0gm., n = 5 and H:45.4 +/- 1.4gm, n = 5; pN.S.) The results suggest that an adaptive response to changes in GFR is the predominant cause of the decrease in RNa and renal QO2 under hypothyroid conditions. A modest reduction in renal tubular sodium reabsorptive capacity in hypothyroid animals is probably secondary to a direct effect of TH on active sodium transport.

Lowered respiratory response to adenosine diphosphate of mitochondria isolated from a mutant B strain of Schizophyllum commune

Mitochondria were isolated from the mycelium of a B-factor mutant in Schizophyllum commune that was previously shown to have its energy metabolism partially uncoupled. These mitochondria were compared to mitochondria isolated from wild-type mycelia and were found to increase their respiration rate upon addition of adenosine diphosphate to only one-half the extent of the mitochondria of wild-type mycelia.

Biochemistry of hyperthyroidism and hypothyroidism

The thyroid hormones act directly on mitochondria, and thereby control the transformation of the energy derived from oxidations into a form utilizable by the cell. Through their direct actions on mitochondria, the hormones also control indirectly the rate of protein synthesis and thereby the amount of oxidative apparatus in the cell. A rationale for the effects of thyroid hormone excess or deficiency is based upon studies of the mechanism of thyroid hormone action. In hypothyroidism, slow fuel consumption leads to a low output of utilizable energy. In hyperthyroidism, rapid fuel consumption leads to a high energy output, but as efficiency decreases, the utilizable energy produced decreases. Many of the chemical and physical features of these diseases can be reduced to changes in available energy.